A Local Area Network (commonly referred to as LAN) permits data transmission and sharing of such information between discrete terminal devices such as computer terminals, peripheral equipment and similar work stations. Reference to the form and function of a typical system is shown in U.S. Pat. No. 4,775,864 dated Oct. 4, 1988 can be made for an explanation of an ilustrative LAN and connections thereto.
Many LANS employ coaxial cable or twisted lines to interconnect the various nodes and hubs to which the terminal devices. Ideally one would want to be able to link as many devices to the LAN as one needs in a given network. The problem, however, is that as each device is coupled into the LAN, it places an output impedance in parallel across the coaxial cable or lines which interconnect and form the LAN. As devices are added to the LAN, these parallel impedances combine to reduce the presented impedance of the LAN interconnecting cable. A point is reached where the actual impedance of the interconnecting LAN line is brought so low that severe mismatch is created on this line. This leads to ringing, pulse (data) distortion, and other unwanted effects.
There are many circuits which are designed to act as an interface between a given device and the LAN. Most often these circuits employ complex arrangements and circuitry to maintain a high impedance when coupled to the interconnecting cables of the LAN. When the interface device presents a significantly higher impedance than the characteristic impedance of the LAN interconnecting cable many such interface devices can be placed across the LAN cable before a serious effect on the measured impedance of same is noticed. While such high impedance interface apparata is available, often times they involve complicated circuitry. This leads to considerable expense in implementation and attendant reliability considerations.